Hi I would like to know the maximum amount of columns allowed per table for the different storage engines and the max row size. I searched the mariadb website documentation and could not find the information. Thank you
MariaDB in its current form is still close enough to MySQL that the same limits apply. The MariaDB fork may diverge further as time goes on.
The actual answer for the maximum number of columns per table in MySQL is complex, because of differences in data types, storage engines, and metadata storage. Sorry there's not just a simple answer.
As #Manquer cites, there's an absolute limit of 64KB per row, but BLOB/TEXT columns don't count toward this limit.
InnoDB pages must fit at least two rows per page, and a page is 16KB minus some header information. So regardless of number of columns, the row must be about 8000 bytes. But VARCHAR/BLOB/TEXT columns can overflow to additional pages in interesting ways. See http://www.mysqlperformanceblog.com/2010/02/09/blob-storage-in-innodb/ for the details.
But there are even further restrictions, based on the .FRM metadata file that applies to all storage engines. It gets really complex, read http://www.mysqlperformanceblog.com/2013/04/08/understanding-the-maximum-number-of-columns-in-a-mysql-table/ for the details and examples about that. There's too much to copy down here.
Given the latter blog article, I was able to design a table that failed at 59 columns.
MariaDb being originally a fork and drop in replacement for MySQL, mostly follows similar design constraints as MySQL. Although MariaDB documentation does not explicitly say how many columns are allowed.
This number is highly dependent a number of factors including the storage engine used and the way the columns are structured. For InnoDB this is 1,000.
See explanation below from the official documentation (Ref: Column Count Limit)
There is a hard limit of 4096 columns per table, but the effective
maximum may be less for a given table. The exact limit depends on
several interacting factors.
Every table (regardless of storage engine) has a maximum row size of
65,535 bytes. Storage engines may place additional constraints on this
limit, reducing the effective maximum row size.
The maximum row size constrains the number (and possibly size) of
columns because the total length of all columns cannot exceed this
size.
...
Individual storage engines might impose additional restrictions that
limit table column count.
InnoDB permits up to 1000 columns.
This applies to MariaDb as well
Related
Is it possible to get the amount of space on disk that a particular table uses? Let's say I have a million users stored in my table and I want to know how much space it's required to store all users and/or one of them.
Update:
I'm planning to use redis to cache some fields from one particular table in memory to quickly retrieve the needed data after. So I need to calculate how much space approximately will it take and thus will it fit in the memory or not. Definitely it depends on the data types that I use inside my table but if a table consists of several dozens of fields it would take too much time to count this one by one.
There is exactly such answer for the MySQL though it's not suitable for SQL Server: How can you determine how much disk space a particular MySQL table is taking up? You can check it to see what I mean.
If you have SSMS, you can right-click on the table in the Object Explorer, go to Properties, and then look at the Storage page. The field, Data space, is the size of the data in that table, but it probably does not include some of the overhead costs of the table.
This is really an extended comment, because it does not directly answer the question.
For most purposes, you just use the size of the columns, add them together, and multiply by the number of rows. This lowballs the estimate, but it is reasonable. And (depending on how you handle the types) might be a reasonable estimate of the size of exporting the data.
That said, the storage of tables is a difficult matter. Here are some of the factors you need to take into account:
The size of individuals fields. This is made slightly more difficult because some types have varying sizes, so those are entirely data dependent.
The number of pages occupied by a table (or equivalently how full each data page is). Note that this can vary, depending on how full each table is.
The number of pages occupied by "overflow" data types, such as varchar(max).
Whether or not the data pages are compressed or encrypted.
The indexes for the table.
How full each index page is.
And, no doubt, I've left out a bunch of other relevant internal details (here is a place to start on page layouts).
In other words, there isn't a simple answer. Equivalent tables on two different systems could occupy very different amounts of space. This is true of the "same" table on the same system at different times.
The general answer when working with databases is that you need a lot more space than number of rows * row size -- I seem to recall using a factor of 3 at one point in time. In general, storage is pretty cheap, so this is not the limiting factor using a database.
We would need to see your full database schema, with tables and columns and all fields' data types. Without those pieces of information it's just a lucky guess. Here is a helpful cheat sheet of the sizes of each data type: https://www.connectionstrings.com/sql-server-2012-data-types-reference/
Then you just have to do the Math and calculate the space needed for X, which is your number of records
What is the maximum number of results possible from the following SQL query for DB2 on z/OS?
SELECT NAME FROM SYSIBM.SYSTABLES WHERE TYPE='T' AND CREATOR=? ORDER BY NAME ASC
This query is intended to fetch a list of all table names under a specific schema/creator in a DB2 subsystem.
I am having trouble finding a definitive answer. According to IBM's "Limits in DB2 for z/OS" article, the maximum number of internal objects for a DB2 database is 32767. Objects include views, indexes, etc.
I would prefer a more specific answer for maximum number of table names under one schema. For instance, here is an excerpt from an IDUG thread for a related question:
Based on the limit of 32767 objects in one database, where each tablespace takes two entries, and tables and indexes take one entry each, then the theoretical max would seem to be, with one tablespace per database,
32767 - 2 (for the single tablespace) = 32765 / 2 = 16382 tables, assuming you need at least one index per table.
Are these assumptions valid (each tablespace takes two entries, at least one index per table)?
assuming you need at least one index per table.
That assumption doesn't seem valid. Tables don't always have indexes. And you are thinking about edge cases where someone is already doing something weird, so I definitely wouldn't presume there will be indexes on each table.*
If you really want to handle all possible cases, I think you need to assume that you can have up to 32765 tables (two object identifiers are needed for a table space, as mentioned in the quote).
*Also, the footnote in the documentation you linked indicates that an index takes up two internal object descriptors. So the math is also incorrect in that quote. It would actually be 10921 tables if they each had an index. But I don't think that is relevant anyway.
I'm not sure your assumptions are appropriate because there are just too many possibilities to consider and in the grand scheme of things probably doesn't make much difference to the answer from your point of view
I'll rephrase your question to make sure I understand you correctly, you are after the maximum number of rows i.e. worst case scenario, that could possibly be returned by your SQL query?
DB2 System Limits
Maximum databases
Limited by system storage and EDM pool size
Maximum number of databases
65217
Maximum number of internal objects for each database
32767
The number of internal object descriptors (OBDs) for external objects are as follows
Table space: 2 (minimum required)
Table: 1
Therefore the maximum number of rows from your SQL query:
65217 * (32767 - 2) = 2,136,835,005
N.B. DB2 for z/OS does not have a 1:1 ratio between schemas and databases
N.N.B. This figure assumes 32,765 tables/tablespace/database i.e. 32765:1:1
I'm sure ±2 billion rows is NOT a "reasonable" expectation for max number of table names that might show up under a schema but it is possible
I have a multiple tables in my application that are both very wide and very tall. The width comes from sometimes 10-20 columns with a variety of datatypes varchar/nvarchar as well as char/bigint/int/decimal. My understanding is that the default page size in SQL is 8k, but can be manually changed. Also, that varchar/nvarchar columns are except from this restriction and they are often(always?) moved to a separate location, a process called Row_Overflow. Evenso, MS documentation states that Row-Overflowed data will degrade performance. "querying and performing other select operations, such as sorts or joins on large records that contain row-overflow data slows processing time, because these records are processed synchronously instead of asynchronously"
They recommend moving large columns into joinable metadata tables. "This can then be queried in an asynchronous JOIN operation".
My question is, is it worth enlarging the page size to accomodate the wide columns, and are there other performance problems thatd come up? If I didnt do that and instead partitioned the table into 1 or more metadata tables, and the tables got "big" like in the 100MM records range, wouldnt joining the partitioned tables far outweigh the benefits? Also, if the SQL Server is on a single core machine (or on SQL Azure) my understanding is that parallelism is disabled, so would that also eliminate the benefit of moving the tables intro partitions given that the join would no longer be asynchronous? Any other strategies that you'd recommend?
EDIT: Per the great comments below and some additional reading (that I shouldve done originally), you cannot manually alter SQL Server page size. Also, related SO post: How do we change the page size of SQL Server?. Additional great answer there from #remus-rusanu
You cannot change the page size.
varchar(x) and (MAX) are moved off-row when necessary - that is, there isn't enough space on the page itself. If you have lots of large values it may indeed be more effective to move them into other tables and then join them onto the base table - especially if you're not always querying for that data.
There is no concept of synchronously and asynchronously reading that off-row data. When you execute a query, it's run synchronously. You may have parallelization but that's a completely different thing, and it's not affected in this case.
Edit: To give you more practical advice you'll need to show us your schema and some realistic data characteristics.
My understanding is that the default page size in SQL is 8k, but can
be manually changed
The 'large pages' settings refers to memory allocations, not to change the database page size. See SQL Server and Large Pages Explained. I'm afraid your understanding is a little off.
As a general non-specific advice, for wide fixed length columns the best strategy is to deploy row-compression. For nvarchar, Unicode compression can help a lot. For specific advice, you need to measure. What is the exact performance problem you encountered? How did you measured? Did you used a methodology like Waits and Queues to identify the bottlenecks and you are positive that row size and off-row storage is an issue? It seems to me that you used the other 'methodology'...
you can't change the default 8k page size
varchar and nvarchar are treated like any other field, unless the are (max) which means they will be stored a little bit different because they can extend the size of a page, which you cant do with another datatype, also because it is not possible
For example, if you try to execute this statement:
create table test_varchars(
a varchar(8000),
b varchar(8001),
c nvarchar(4000),
d nvarchar(4001)
)
Column a and c are fine because both on them are max 8000 bytes in length.
But, you would get the following errors on columns b and d:
The size (8001) given to the column 'b' exceeds the maximum allowed for any data type (8000).
The size (4001) given to the parameter 'd' exceeds the maximum allowed (4000).
because both of them exceed the 8000 bytes limit. (Remember that the n in front of varchar or char means unicode and occupies double of space)
For example, a website offers the ability to create mobile surveys. Each survey ID is a FK in the survey response table, which contains ALL of the survey responses.
What is the size limitation of this table in a SQL Server 2008 db, if the table contains, say 20 varchar(255) fields including the bigint PK & FK?
I realize this would depend on the file size limitation as well, but I would like some more of an educated answer rather than my guess on this.
In terms of searchability, some fields that contain geo-related details such as the survey ID, city, state, and two commends fields would have to be searchable, and thus indexed ... index only these fields?
Also, aged responses would expire after a given amount of time - thus deleted from the table. Does the table, at this point being very large, need to be re-indexed/cleaned up, after the deletions (which would be an automated process)?
Thanks.
Maximum Capacity Specifications for SQL Server
Bytes per row: 8,060
Rows per table: Limited by available storage
Note
SQL Server supports row-overflow storage which enables variable length
columns to be pushed off-row. Only a 24-byte root is stored in the
main record for variable length columns pushed out of row; because of
this, the effective row limit is higher than in previous releases of
SQL Server. For more information, see the "Row-Overflow Data Exceeding
8 KB" topic in SQL Server Books Online
You mention 'table size' -- does this mean number of rows?
Maximum Capacity Specifications for SQL Server
Rows per table : Limited by available storage
As per this Reference, the max size of a table is limited by the available storage.
It sounds like you are going to have a high traffic and high content table. You should consider performance and storage enhancements like Table Partitioning. Also, because this table will be the victim of often INSERTS/UPDATES/DELETES, carefully plan out your indexing, as indexes add overhead for DML statements on the table.
I'm about to add a new column to my table with 500,000 existing rows. Is there any harm in choosing a large value for the varchar? How exactly are varchars allocated for existing rows? Does it take up a lot of disk space? How about memory effects during run time?
I'm looking for MySQL specific behavior details not general software design advises.
There's no harm in choosing a large value for a varchar field. Only the actual data will be stored, and MySQL doesn't allocate the full specified length for each record. It stores the actual length of the data along with the field, so it doesn't need to store any padding or allocate unused memory.
Depends on what you're doing. See the relevant documentation page for some of the details:
http://dev.mysql.com/doc/refman/5.0/en/char.html
The penalty in disk space isn't really any different than what you have for e.g. TEXT types, and from a performance perspective it MAY actually be faster.
The primary problem is the maximum row size. Note that the exact implications of this differ between storage engines. Consult the MySQL docs for your storage engine of choice for maximum row size information.
I should also add that there can be performance benefits to minimizing row size, but it really depends on your workload, indexing, and just how big the rows are, whether or not it will be meaningful for you.
MySQL VARCHAR fields store the contents, plus 2 bytes for length. So empty VARCHAR fields will use up space to mark their lengths.
Also, if this is the only VARCHAR field in your table, and your storage engine is MyISAM, it would force dynamic row format which may yield a performance hit (testing will confirm).
http://dev.mysql.com/doc/refman/5.0/en/column-count-limit.html
http://dev.mysql.com/doc/refman/5.0/en/dynamic-format.html